Function.prototype.apply = function(obj,arr){
    obj = obj || window
    obj.fn = this;
    let args = [];
    if(!arr){
        obj.fn();
    }else{
        arr.forEach(function(item,i){
            args.push('arr['+i+']')
        })
    }
    eval('obj.fn('+args+')')//"obj.fn(arr[0],arr[1])" ,数组与字符串链接相当于执行了数组的join()
    delete obj.fn;
}